The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A method of intensification of oil or gas production by means of hydraulic fracturing of oil or gas-bearing formations has been known since 1940. Since then, a lot of solid fillers (proppant) have been proposed to be used for fixing the fractures produced. For this purpose, it was proposed to use sand, nutshells, aluminum and its alloys, wooden chips, crushed coke, crushed coal, mullite, glass balls, zirconium dioxide, silicon nitride and silicon carbide. Among the above-mentioned substances proposed as proppant, sand has had the widest practical use. However, sand is a low-strength material and it is difficult to use sand at the oil and gas occurrence depths where pressures exceed 34.5 MPa, because of the destruction of the sand. In view of this, medium-strength and high-strength proppant made of sintered bauxites were proposed to be used at great oil and gas occurrence depths.
Thus, there are known methods for producing propping agents from sintered bauxites containing up to 85-88 wt % of Al2O3 (U.S. Pat. No. 4,068,718, 1978; EP, Patent 0112360, 1984). A proppant made of sintered bauxite is a high-strength material. However, the cost of this proppant is much higher than that of the sand which is still widely used due to its low cost, in spite of its disadvantages. Besides, the density of proppant made of sintered bauxites exceeds 3 g/cm3.
Less expensive clay materials and minerals were proposed to be used as the feedstock, so as to reduce the cost and the density of proppant while maintaining their strength properties.
Thus, there is a known proppant and its production method (EP, Patent 0168479, 1989) which involves the production of particles from minerals and the saturation of the surface of spheres with Al2O3. The proppant produced by this method contains 50 to 97 wt % of the mineral and 3 to 50 wt % of Al2O3. The minerals are selected from the group containing nepheline syenite, basalt, feldspar, shale, argillite, pyroxene and their mixtures. The proposed proppant has a lower cost, lower sintering temperature and lower gravity as compared with the proppant made of sintered bauxite. However, the cost of this proppant is still much higher than that of the proppant made of sand, as the feedstock (i.e. the minerals and Al2O3 used in the proppant production) is still rather expensive.
There is a known proppant (U.S. Pat. No. 4,522,731, 1985) which was proposed to be produced from alumina ore containing over 5 wt % of gibbsite (Eufaula, Ala.). According to the applicant, this proppant has a lower density as compared with the sintered bauxite, and has a mechanical strength which is capable of withstanding pressures up to 70 MPa and higher pressures. The proppant particles may include strengthening additives selected from the group containing nepheline syenite, fused bauxite, wollastonite, talc and fluorspar which can be added, in the amount up to 5 wt %, to the alumina ore during the production of spherical particles. However, according to the known patent, the proppant particles are sintered in a fluidized bed, which is not always economically feasible.
There is a known propping agent (U.S. Pat. No. 4,668,645, 1987) produced from inexpensive bauxite clay from Arkansas. It was established that it was possible to produce the end product (proppant) from the feedstock containing 16 to 19 wt % of silica and less than 0.35 wt % of oxides of alkali metals and alkali-earth metals, and that this proppant would meet the requirements imposed on medium-strength propping agents. According to the method described in the patent, the feedstock containing 70 to 80 wt % of aluminum oxide is calcined at a temperature of 1,000° C. until water has been completely removed. Then, it is reduced to powder with a degree of fineness below 10 microns, formed into spherical granules in an intense mixer and sintered at a temperature of 1,400 to 1,500° C. until the calcined material has converted into mullite and corundum. However, the use of the feedstock having a low content of oxides of alkali metals and alkali-earth metals and a relatively high content of aluminum oxide in this method limits considerably the sources of raw materials for the propping agent production.
There is a known proppant (U.S. Pat. No. 4,427,068, 1984) which was proposed to be produced from one or more clays, with additions of bauxites, alumina oxides or their mixtures. The sintered spherical granules of the end product have an Al2O3/SiO2 ratio varying from 9:1 to 1:1 and a density below 3.4 g/cm3. In this method, diaspore, refractory and flint clays were used and their content in the initial mixture of primary materials was at least 40 wt %. In accordance with the invention under this patent, powdered annealed clay and Al2O3 (or bauxite and their mixtures) were mixed together and granulated in an intense mixture, and the resultant granules were sintered at a temperature of 1,500° C. after drying. This proppant production method does not impose any strict limitation on the feedstock composition. However, when using the above clays, it is necessary to add up to 40 wt % of expensive bauxite to these clays, which increases considerably the cost of the feedstock and, consequently, the cost of the end product.